This experiment is designed to determine by histopathological and ultrastructural techniques the effects of argon laser photocoagulation on the retina and optic nerve. Multiple argon laser photocoagulation lesions of varying intensity and size will be applied onto subhuman primate retinas. Fluorescein angiographic, light microscopic, and ultrastructural alterations within the retinas will be analyzed in order to determine the basic nature of the induced lesions and to support (or refute) the various rationales regarding the clinical use of this instrument in vascular retinopathies. Special emphasis is given to 1) the efficiency of argon burns in vessel closure or obliteration; 2) factors related to movement of fluid between various retinal and choroidal components; and 3) probably most important, a thorough analysis of laser burn effects on the retinal nerve fiber layer and the optic nerve parenchyma. This analysis relates directly to the possibility of iatrogenic damage from the argon laser, and has not heretofore been sufficiently evaluated.